1. Field of the Invention
The invention can be applied particularly advantageously in the automotive field, especially as regards the injection of fuel into a combustion chamber.
2. Description of Related Art
From the prior art there is known a first type of injection devices designated as recessed needles. In this traditional configuration, each injector is provided with a jet needle that is capable of being displaced axially. This mobility exists between a closed position, in which the distal end of the jet needle blocks an aperture intended for ejection of the fluid, and an open position, in which the said distal end is positioned at a distance from this same aperture.
It must also be noted that it is immaterial whether the aperture comprises a single orifice or a plurality of holes arranged downstream from the seat, which is intended to cooperate sealingly with the distal end of the jet needle. This latter configuration proves particularly adapted to the injection of liquid, since the presence of orifices in large numbers is of such nature as to perturb the ejection of the liquid and consequently to multiply the droplets.
Be that as it may, this type of recessed-needle injector suffers from the disadvantage that it functions almost by the all-or-nothing principle. In other words, the needle tip either allows a maximum quantity of pressurized fluid to pass, or it prevents it from escaping via the aperture. Thus the parameters for adjustment of such a system are essentially limited to the fluid pressure and to the discharge cross section of the aperture.
In actual practice, and especially in the case in which the fluid is a liquid, a recessed-needle injector almost always functions at constant pressure. Thus it is the diameter of the holes that will determine the size of the droplets. Since the dimensions of the holes in question are fixed by design, however, it appears to be particularly difficult to modify the droplet size at all. Even if the recessed needle is opened and closed very rapidly, the perturbation then created will be very largely insufficient to generate a mist of small diffuse droplets effectively.
Thus, with an injection device of the recessed-needle type, it is certainly possible to control the quantity of fluid injected, but from all evidence it is not conceivable that the size and diffusion of droplets can be controlled precisely. As is well known, this constitutes a major drawback in terms of efficiency.
Another important parameter to be controlled concerns the minimum quantity that it is possible to inject. As it happens, in a large proportion of recessed-needle injectors, the fluid overpressure is used to move the jet needle from its closed position to its open position. The response time of the system then depends on the magnitude of the pressure in question. In practice, it is necessary to raise the fluid pressure if it is desired to shorten the opening time of the recessed needle, but then the minimum quantities injected are increased. This constitutes a further disadvantage for this type of system.
A second type of prior art injection devices, designated as protruding needles, makes it possible to alleviate these difficulties. Here, each injector is provided with a kind of valve composed of a stem, one end of which forms a poppet and is capable of cooperating by leaktight contact with a seat defining a fluid-ejection aperture. As in the preceding case, the poppet stem formed in this way is mounted to be mobile by axial displacement between a closed position, in which the poppet blocks the aperture, and an open position, in which the said poppet is positioned at a distance from the said aperture.
The mobility of such a poppet stem is generally achieved by using either a piezoelectric actuator or a magnetostrictive actuator. Specifically, this consists in coupling the poppet stem of the injector with an appended element advantageously composed of a material known as active, meaning capable of changing shape, and especially of growing longer, when either an electric current or a magnetic field respectively is passed through it. Since the corresponding physical principles and the modes of operation of such actuators are fully known, they will not be further described here. It will be recalled simply that the assembly is generally arranged in such a way that excitation of the active material, electrical or magnetic respectively, causes elongation of the appended element and consequently a displacement of the poppet stem in its entirety. The distal end of the poppet stem is then no longer in contact with its seat, so that the fluid under pressure can then escape via the aperture.
Compared with their recessed-needle homologs, the protruding-needle injectors have the advantage of being able to have a variable lift at the poppet level. At constant pressure, therefore, it is possible to have a discharge cross section that is variable in time. For example, in the case of a piezoelectric actuator, a given elongation of the active material will be obtained as a function of the voltage applied to the appended element. The corresponding elongation of the appended element then causes a proportional displacement of the poppet stem, and consequently an equally proportional lift of the poppet.
Nevertheless, the injection devices of the protruding-needle type suffer from disadvantages that are peculiar to them.
With an appended element of piezoelectric material, it is possible to achieve deformations on the order of one thousandth, or in other words approximately 10 μm of displacement per 10 mm of piezoelectric stack. That means that the appended element must be extremely long if it is wished to obtain a poppet displacement of traditionally 50 μm. That then implies the need to charge very large capacitances, on the order of 3 to 3.5 μF, for a 30-mm stack, for example. Consequently, electronics of considerable power are needed if it is wished to shorten the switching time.
In addition, during operation of the protruding-needle injector, the great length of the appended element then constitutes a disadvantage in terms of weight. The assembly composed of the poppet stem, of the appended element and of other means for elastic return, then constitutes a relatively large mobile mass. The resulting significant inertia will then retard the responsiveness of the piezoelectric material even more.
Thus, by reason essentially of the large capacitances to be charged and of the large mass to be displaced, the injection devices having protruding needles and piezoelectric actuators prove to be intrinsically limited in terms of response time.
With an appended element of magnetostrictive material, it is essentially the problem of system inertia that constitutes the main handicap. After all, it must not be forgotten that the mobile mass being displaced is very large with prior art injection devices, since it corresponds to the combined masses of the stem, which is often long, and of the associated poppet.